I think it is MKB Kursaal in Oostende. You can check with photo of this forum page (viewtopic.php?t=127943) ; Look at the building in background right.
Thanks to both for your information, Sébastien and ludvig...ludvig wrote
It is an old German naval gun , 10,5 cm SKL/40
Here, another recent image from Ebay. I am not sure if an 38cm SK C34 gun turret of Todt batterie (as I think) or an 40.6cm gun turret of Lindemann batterie....Somebody ?